all these stories of "a mate of a mate down the pub said..." make me laugh. Who honestly gives a shit what cars a g40 is faster than or not? And don't forget, these 240bhp or whatever cars might have been driven by idiots or (often the case!) people who didnt think they were even in a "race". Just go and test drive one and if you like it, buy it. If you are wanting the fastest car on the planet then it's not a g40.
Truth is a totally standard g40 isnt really that fast, especially in comparison with more modern hot hatches. And the 1970s design chassis certainly doesnt help, even with a simple straight line drag race, the front wheels are moving around loads.
But their beauty is that you dont have to do much to make them pretty crazy. 65mm pulley, chip, sort the air box out and you've got a bloody good fun car that will hold it's own against much more modern stuff and cars that on paper, should be quicker.